1. Technical Field
This invention relates generally to bearings for locomotive engines, and more particularly to methods for detecting abnormal wear of locomotive engine bearings to permit corrective action to be taken. The disclosure incorporates bismuth tracer bearings disclosed in U.S. provisional patent application No. 60/113,775, filed Dec. 23, 1998, whose priority is claimed for this application.
2. Related Prior Art
Multi-layer plain engine bearings are used in locomotive engine applications to journal working components of the engine, such as the crankshaft. The bearings are made with a rigid metal backing on which a layer of functional copper-tin bearing metal is applied. The bearing layer is plated with a diffusion barrier layer of nickel and then plated with a softer overlay metal which serves as the running surface of the shaft. Under normal operating conditions, the overlay will survive for considerable time before the underlying bearing layer is exposed. However, should a bearing be improperly loaded due to, for example, misalignment between the shaft and engine block, there is the possibility of accelerated wear of the overlay, causing the bearing layer to become exposed too soon, and thus lessening the operating life of the bearing.
In one known attempt to monitor the wear characteristics of engine bearings in truck applications, a layer of silver was plated onto the nickel barrier layer of a sliding bearing, and the silver layer in turn covered by another layer of nickel, prior to plating the overlay. With such a bearing installed, the engine oil was periodically tested for the presence of silver. If silver was detected in the engine oil too soon (i.e., before the time in which the overlay would be expected to wear under normal operating conditions) then the conclusion was drawn that one or more bearings must be wearing prematurely, and the engine would be torn down to correct the problem in advance of damage to the engine.
However, in locomotive engine applications, the engine oil typically would be exposed to sources of silver apart from the silver tracer layer of such engine bearings. For example, the slipper bearings of the connecting rods of some engines contain silver which could leach into the engine oil and give a false indication of improper wear of the engine bearings. It is desirable, of course, to avoid the tremendous time and expense of tearing down a locomotive engine to replace the engine bearings and correct misalignments, etc., if such is not necessary.
Another drawback to the use of such a silver tracer layer is that it adds cost and complexity to the manufacture of locomotive engine bearings. Special requirements must be made for plating the silver and an added nickel layer is required to sandwich the silver between two layers of nickel. Environmental considerations must also be taken into account when plating silver. For these reasons, the use of a silver as a tracer material in locomotive engines is not satisfactory.
One of the objects of the present invention is to provide a method of detecting the wear characteristics of engine bearings for locomotive engine applications that minimizes or eliminates the foregoing disadvantages of silver.
According to the invention, a method is provided for detecting the wear characteristics of a multi-layer plain engine bearing in a locomotive engine having engine oil free of exposure to any sources of bismuth apart from the engine bearing. The method includes fabricating a rigid metal backing for the plain bearing and applying a functional bearing layer of copper-based bearing metal to the backing having bismuth incorporated therein. An overlay of a relatively softer metal is applied over the bearing layer and is free of bismuth. The bearing is installed in the locomotive engine and the engine oil is periodically tested for the presence of bismuth. If it is determined that bismuth is present in the engine oil too soon relative to the life expectancy of the overlay, then such would indicate premature wear of the overlay enabling corrective action to be taken to protect the engine.
Various locomotive engines have been analyzed and it was discovered that all were free of any source of bismuth, thus avoiding the problem of false positives associated with silver. Further, unlike silver, the bismuth can be incorporated into the traditional bearing metal alloys without having any detrimental effect on their properties, and perhaps enhancing their sliding properties.